Our research work has been concerned with the relationship between the membrane transport of ions and the role of ions in the regulation of metabolic activity. Specifically, we have been studying the function of the plasma membrane of the Ehrlich mouse ascites tumor cell in the control of inorganic phosphate (Pi) transport and the relationship of this transport to energy metabolism. Pi is not passively distributed across the membrane. From competition studies with arsenate it is clear that the Pi-transport system favors Pi over arsenate by a factor of 7. An analysis of the kinetics indicated that Pi-transport was mediated by a carrier, and that the free carrier crosses the membrane at 5 percent of the rate of the carrier-Pi complex. Our most recent experiments have been concerned with the role of positive fixed membrane charge in anion transport. Sulfate transport is, while chloride transport is not, readily explainable in terms of Passow's fixed charge hypothesis. Experiments not in progress are designed to determine whether the saturation-type kinetics exhibited by the Pi transport system is interpretable in terms of membrane charge density. Agents known to react specifically with surface charged groups on the membrane are also being examined for their effects on Pi (anion) transport.